The purpose of this study is to investigate in pneumococcus the structure of the DNA determining sulfonamide resistance and resistance to streptomycin and other ribosome-modifying drugs. Numerous mutants, linked and unlinked, are available in this region of the pneumococcal genome, as well as a unique diploid region which carries several of these resistance sites. Genetic fine structure is investigated in connection with DNA transformational exchange of segments of this DNA, and also in the recombinational exchanges that occur within the partial diploid strains. The effect of mild shear and nucleolytic degradation of the DNA is studied in the former process as a means of dissecting small regions of the structure.